This invention relates generally to the field of internal combustion engines and more particularly to the cooling systems used to control the heat generated by such combustion engines. Most particularly, this invention relates to thermostats used to control the flow of the coolant between an engine and a heat exchanger such as a radiator.
Thermostats have been known and used extensively to control the circulation of coolant in internal combustion engines. In the past, the thermostats have taken the form of valves which are immersed in the coolant in, for example, a coolant conduit. Most commonly the valves include a valve member which spans the conduit and sits against a valve seat. Thus, in the closed position the valve substantially blocks the flow of coolant, for example, to the radiator, allowing the coolant to re-circulate past the engine and to heat up more quickly.
Typically such valves include a closed body containing a thermally expandible material such as wax. A piston is provided which is thrust outward upon the expansion of the wax due to higher coolant temperatures. The piston lifts the valve off the valve seat to allow the coolant to circulate past a heat exchanger, such as a radiator. This lowers the temperature of the coolant and removes heat from the engine. A spring is provided to urge the valve to a closed position so that in the resting or cooled state the valve is closed. Thus, when an engine is first started, the valve will be closed allowing the engine to attain its optimum running temperature more quickly.
Thermostats, to date, have been designed to permit the engine to operate over time at a constant optimum temperature. The thermostat accomplishes this by opening a valve in the cooling system when the engine temperature, and thus the liquid coolant temperature, rises. Opening the valve permits more flow to a heat exchanger such as a radiator, permitting more heat to be dissipated, which in turn can lower the engine temperature. As the engine temperature drops, and thus the coolant temperature drops, the valve closes, reducing the amount of heat dissipated and again maintaining an optimum operating temperature.
Such prior art thermostats are effective, simple and reliable, but suffer from several drawbacks. One is that the thermostat essentially requires the engine designer to set one optimum engine temperature. However, in practice, the engine operating temperature is known to affect engine performance. Specifically, a hotter running engine produces less in the way of emissions, by permitting more complete combustion which in turn improves fuel economy. A hotter running engine will deliver less power, while a cooler running engine delivers more power. Thus, any single optimum engine temperature is a compromise between power and emissions.
Another drawback is that thermostats are slow to respond. The coolant temperature change is fairly gradual and since the change in coolant temperature controls movement of the piston, the valve only opens slowly. Essentially the response of the thermostat lags the engine demand and thus acts as a dampened system. For example, it might take the thermostat 12 minutes to respond in winter when the engine start is very cold, and about 5 minutes in summer where the engine start temperature is warmer. Sharp changes in engine temperature which arise and then recede quickly are not well managed by the thermostat. However, such sharp changes may occur, for example during acceleration from a stop, when accelerating to pass, or when climbing a hill. Therefore there has been an effort to develop a thermostat which responds, on demand, rather than simply following coolant temperature. Of course, the thermostat still needs to reliably respond to coolant temperature changes in a manner which prevents overheating.
Various levers and actuators have been proposed to open and close valve elements on demand, but these suffer from various disadvantages. Firstly, they are relatively expensive. Secondly, they involve complex moving parts, which can fail over time. A failed system could lead to overheating and failure of the engine, which is unacceptable. Thus, electromechanical systems are inappropriate for the under the hood environment.
U.S. Pat. No. 4,890,790 and its related U.S. Pat. No. 4,961,530 disclose a better thermo-mechanical solution with a thermostat which is more responsive than one limited to responding to coolant temperature only. This patent teaches a first thermostat 40 located in the usual position within a coolant conduit and then a second thermostat like device 52 (called a thermal motor) located outside of the conduit and being insulated therefrom. The device 52 includes the same element as a thermostat as previously described, namely a closed body, a thermally expandible material within the body and a piston which can be extended in response to a temperature rise in the thermally expandible material. However, rather than the coolant temperature governing the degree of extension, the device 52 includes a small electrical heater within the closed body which can be used to heat the expandible material to in turn cause a piston to extend. The pistons of device 52 and the regular thermostat are made coaxial so that when the electrically controlled piston extends, the valve of the thermostat is lifted off the valve seat. The patents teach that in this way the valve can be opened in response to engine parameters such as load or the like measured by other sensors and the coolant allowed to circulate before the heat builds up in the engine. This ability to control the opening of the valve is said to virtually eliminate customer complaints of engine overheating and improves fuel economy and reduces emissions.
While a reasonable solution in some respects, this prior art device still suffers from numerous drawbacks and has not found widespread acceptance. For example, the thermal motor 52, although insulated from the coolant, projects, somewhat exposed, into the under the hood compartment. The air temperature of the under the hood environment can vary widely, depending upon outside temperature, and further can be quite hot when the engine reaches steady state operating temperatures, up to about 25% higher than the coolant temperature. Such a wide temperature range for the operating conditions of the thermally activated motor make it difficult to predict how much heat is needed from the electrical heater to cause the motor to move. Worse, the device 52 might be activated by the ambient temperature without even being controlled by the engine control system, which is unacceptable.
Further, connecting the piston of the device 52 to the thermostat piston coaxially magnifies the effect of the two thermally activated pistons systems since their movements are cumulative. This makes the valve opening and closing overly sensitive and difficult to reliably control. What is believed to happen in practice is that the valve will tend to open too much and then close too much and to essentially oscillate about the desired set point in an undampened manner. Such oscillation is hard on the components and renders the desired temperature less of the time, making the device less efficient rather than more efficient. What is needed therefore is a simple and reliable way of providing accurate temperature control for an internal combustion engine which responds both to the coolant temperature, which is responsive to engine load and which avoids these problems.
What is needed therefore is a controllable thermostat system which on the one hand is readily controlled by an engine control system to permit rapid response to short duration peak loads and yet which still responds in a safe and reliable way to changes in coolant temperature to prevent overheating. In this way, in the event the device ever fails, the thermostat portion will still be active to prevent engine overheating. Further the system should be made from inexpensive components which are reliable, safe and simple to install. The system should respond appropriately and not for example be susceptible to changes in operating environment causing the device to undesirably initiate, nor should the device be too sensitive and tend to overshoot in an undampened way any desired set point temperature. Further, the device should permit the engine temperature to be lowered on demand, to deliver more power, but also let the engine operate at high temperatures, to reduce emissions. The device should also respond rapidly to permit the engine temperature to be reduced, for example, within a time horizon of a real time loading event of an engine.
Therefore, according to a first aspect of the present invention there is provided, an apparatus for controlling a temperature of an engine by controlling a flow of a liquid engine coolant, the apparatus comprising:
a thermostat having a temperature responsive valve for substantially blocking and substantially unblocking the flow of said liquid coolant to a radiator, said thermostat having a first temperature activation range;
a thermally activated actuator operatively connected to said valve, said actuator having a second temperature activation range above said first temperature activation range; and
a source of electrothermal energy for activating said actuator to cause said temperature responsive valve to unblock the flow of said liquid coolant on demand.
According to a further aspect of the invention there is provided a method of controlling a temperature of an engine having a coolant circulation system comprising the steps of:
a) providing a thermally activated actuator;
b) providing a thermostat having a fixed thrust surface and an openable valve including by a valve body;
c) operatively connecting said thermally activated actuator to said valve body of said thermostat;
d) monitoring said engine to determine when to open said valve; and
e) opening said valve by activating said thermally activated actuator in response to said engine monitoring.
According to a further aspect of the invention there is provided an apparatus for controlling a temperature of an engine, said apparatus comprising:
a thermostat having a thermally controlled valve which opens to a first position in response to a coolant temperature, said first position corresponding to a first rate of coolant flow sufficient for maintaining an optimum engine temperature;
a thermally controlled actuator for opening said valve to a second position, said second position corresponding to a second rate of coolant flow sufficient to permit said engine to cool to a power delivering temperature below said optimum temperature; and
a heater associated with said actuator, said heater being initiated when additional power is required.